Using arrays for indexing

It is possible to speed things up, using arrays for indexing, instead of looping through the elements an a matrix or vector. Test the following code segment and explain your results:

n = 100000;
a = randn(n, 1);
b = a;
c = a;
loops = 200;

tic
  for j = 1:loops
    for k = 1:length(a)
      if a(k) < 0
        a(k) = 0;
      end
    end
  end
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% Using logical indices instead (b < 0 is an array of 1 and 0, true
% and false).
tic
  for j = 1:loops
    b(b < 0) = 0;
  end 
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% In some applications it is more useful to get the actual indices.
tic;
  for j = 1:loops
    indx = find(c < 0);
    c(indx) = 0;
  end
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You may want to examine how this works when using matrices.